Designing Reactive Bridging O2– at the Atomic Cu–O–Fe Site for Selective NH3 Oxidation

Surface oxidation chemistry involves the formation and breaking of metal–oxygen (M–O) bonds. Ideally, the M–O bonding strength determines the rate of oxygen absorption and dissociation. Here, we design reactive bridging O2– species within the atomic Cu–O–Fe site to accelerate such oxidation chemistry. Using in situ X-ray absorption spectroscopy at the O K-edge and density functional theory calculations, it is found that such bridging O2– has a lower antibonding orbital energy and thus weaker Cu–O/Fe–O strength. In selective NH3 oxidation, the weak Cu–O/Fe–O bond enables fast Cu redox for NH3 conversion and direct NO adsorption via Cu–O–NO to promote N–N coupling toward N2. As a result, 99% N2 selectivity at 100% conversion is achieved at 573 K, exceeding most of the reported results. This result suggests the importance to design, determine, and utilize the unique features of bridging O2– in catalysis.


INTRODUCTION
What happens when a single metal atom is added onto a metal oxide surface? Taking Cu 2+ as an example, the 3d of 4sp of Cu 2+ will first hybridize with the conduction and valence bands of metal oxides, forming four Cu−O−M bonds. 1−4 Due to the different energies of Cu and M valence orbitals, the resultant Cu−O and O−M bonds in the atomic site usually have less bond strength than those in bulk CuO and MO. Those bridging O 2− in Cu−O−M should be more reactive and can be easily taken away by reducing agents compared with lattice O 2− . It is therefore important to probe the electronic structures of those bridging O 2− and rationalize their impact in catalytic reactions. In particular, for oxidation reactions such as CO and NH 3 oxidation, the rapid O 2− removal and formation are the key to activate reductive reactants and molecular O 2 , 5−7 which is ideal for the bridging O 2− due to its weakened bond strength.
Various nitrogen-containing pollutants (NH 3 and NO x ) have raised concerns on public health and environmental protection, which has led to increasingly strict emission standards. As one of the most promising methods for removing ammonia, 8 the selective catalytic oxidation of NH 3 (NH 3 -SCO) to nitrogen has received increasing attention 9,10 and will play a crucial role in the upcoming EU7 standard. However, achieving high activity and nitrogen selectivity simultaneously remains a challenge due to the competing NO desorption pathway before its coupling reaction with NH 3 to form N 2 , as shown below in the internal selective catalytic reduction (i-SCR) mechanism. (2) Having been widely accepted in NH 3 -SCO 11−16 with transition metals, i-SCR includes two steps: First, ammonia is oxidized to NO (eq 1). Second, an N−N bond is formed when the as-prepared NO reacts with unreacted ammonia (eqs 2 and 3). The reaction rate in eqs 1−3 is defined as r1, r2, and r3, respectively. Based on this two-step mechanism, high selectivity toward N 2 can only be achieved under r2 = r1 ≫ r3 conditions. Eq 1 is a typical oxidation reaction promoted by the adsorption of both NH 3 and O 2 with immediate desorption of NO. Noble metals, such as Pt and Pd, generally favor oxygen adsorption and activation, resulting in high oxidation activity. 17,18 However, excessive oxidation leads to low nitrogen selectivity above 300°C in NH 3 -SCO. 19,20 In comparison, Cu is preferred in the selective oxidation reaction due to the weak adsorption of O 2 . 3 As a result, Cu catalysts are commercially used for the reduction of NO with NH 3 . 21−23 For NH 3 -SCO, Cu catalysts are highly active for SCR (eq 2) but suffer from low activity in eq 1. 24,25 Considering this, an enhanced NO formation rate and the subsequent N−N coupling to N 2 are required for Cu catalysts to achieve a high N 2 yield. This can be realized by modifying the reactant and intermediate adsorption behaviors by tuning the electronic structure of the Cu−O−M sites, 26,27 especially the state of bridging O 2− . In addition, the adsorption energy of intermediates is also changed when adsorbing on bridging O 2− . 28

MATERIALS AND METHODS
Catalyst Preparation. The copper-based catalysts were prepared by coprecipitation using copper nitrate trihydrate (Cu(NO 3 ) 2 ·3H 2 O), ferric nitrate nonahydrate (Fe(NO 3 ) 3 · 9H 2 O), and ammonium hydroxide (NH 3 ·H 2 O) as starting materials. In a typical procedure, 2 g of the nitrate precursor (Fe(NO 3 ) 3 ·9H 2 O) and the corresponding mass of Cu(NO 3 ) 2 · 3H 2 O were dissolved in 15 mL of deionized water. Subsequently, the mixture was stirred for 10 min, and then NH 3 ·H 2 O was added dropwise until a pH of 9 was reached under continuous stirring for 10 min. The sample was filtered and washed with deionized water and then dried at 60°C for 12 h. The as-prepared sample was placed in a muffle furnace and calcined at 550°C in air for 4 h. Finally, the sample was slowly cooled to room temperature in the muffle furnace, and the obtained solid was ground to powder.
By changing the loading of Cu, catalysts with different Cu aggregation states were prepared using the above method, which were atomic sites (1 wt % Cu), clusters (20 wt % Cu), and inverse catalysts (70 wt % Cu), respectively. For comparison, pure CuO and Fe 2 O 3 were also prepared using the above method.
The 1 wt % CuO−Fe 2 O 3 (impregnation) was prepared using the wetness impregnation method. Cu(NO 3 ) 2 ·3H 2 O was dissolved in deionized water and then added into Fe 2 O 3 prepared using the above method. The sample was then dried at 60°C for 12 h. Finally, the dried sample was calcined in air at 300°C for 4 h at a heating rate of 5°C/min. 2.1. Ex Situ Characterizations. X-ray diffraction (XRD) measurements were performed using a StadiP diffractometer from STOE with a Mo source (Kα = 0.7093165 Å). The operating voltage and current are 40 kV and 30 mA, respectively. With a resolution of 0.015°each step, the signals of 2θ in the range of 2°−40°were collected.
Scanning transmission electron microscopy (STEM) images of the samples were recorded using the JEM-ARM200CF equipped with bright field (BF) and high-angle annular dark field (HAADF) at 200 keV at the Diamond Light Source. The samples were loaded onto Au grids by sprinkling a small amount of dry sample powder.
The energy-dispersive X-ray spectroscopy (EDX) analysis of the sample was performed using the JEM-ARM200CF equipped with a large solid-angle dual EDX detector. The data were collected in the STEM illumination mode at 200 kV and corrected using special drift correction. Au TEM grids are used to avoid any Cu EDX signals from the grid. Each EDX spectrum image is 100 × 100 pixels in size, with 0.05 s exposure time per pixel.
X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses of the Cu K-edge (8.979 keV) were carried out at the Diamond Light Source (UK), PETRA III DESY (Germany), and SPring-8 (Japan). The samples (≥5 wt % Cu loading) were diluted with boron nitride and pressed into a pellet with a diameter of 1.3 cm for transmission measurements. Samples with 1 wt % Cu loading were directly pressed into pellets for fluorescence measurements. A Cu foil standard was used for energy shift calibration. For the EXAFS evaluation, at least three spectra were merged to improve the signal quality.
The XAFS data were analyzed using the Demeter software package (including Athena and Artemis). 31 Athena software was used for XANES analysis. Artemis software was used to fit the k 2 -weighted (CuO−Fe 2 O 3 ) in real space with 3.0 Å −1 < k < 12.0 Å −1 and 1.0 Å < R < 3.3 Å. The calculated amplitude reduction factor S 0 2 from the EXAFS analysis of the Cu foil was 0.878, which was used as a fixed parameter for EXAFS fitting. The coordination number and bond length were calculated based on the reported structure from the Crystal open database: copper (No. 9013014) and tenorite (No. 1011148).

Near Ambient Pressure (NAP)-NEXAFS Spectroscopy.
In situ NEXAFS experiments for 1 wt % CuO−Fe 2 O 3 were performed at the ISISS beamline of BESSY II in Berlin (Germany). The X-ray was sourced from a bending magnet (D41) and a plane grating monochromator with an energy range from 80 to 2000 eV (soft X-ray range) and a flux of 6 × 10 10 photons/s with 0.1 A ring current using a 111 μm slit and an 80 μm × 200 μm beam spot size. The reaction products were online-monitored using an electron impact mass spectrometer ("PRISMA," Pfeiffer Vacuum GmbH, Asslar (Germany)) connected directly to the main experimental chamber by a leak valve. The pressure in the specimen chamber was precisely controlled (UHV or 0.1−1 mbar) by simultaneous operation of several mass flow controllers for reactive gases and a PID-controlled throttle valve for pumping gas out. Then, 100 mg of catalysts was pressed into pellets with a diameter of 6 mm. The sample pellets (6 mm diameter) were heated uniformly by an IR laser mounted on the rear part of the sample holder. Temperature control was realized by two Ktype thermocouples. The NEXAFS spectra at the Cu L-edge (920−965 eV), Fe L-edge (700−740 eV), O K-edge (510− 560 eV), and N K-edge (390−420 eV) were measured in either the total electron yield (TEY) mode or the Auger electron yield (AEY) mode.

In Situ XANES.
In situ XANES experiments for 1 wt % CuO−Fe 2 O 3 were performed at SPring-8 in Japan. In the experiment, 1 wt % CuO−Fe 2 O 3 was pressed into a pellet and measured at room temperature, 573 and 673 K. XANES spectra of each gas composition were recorded between 6770 and 8160 eV in the transmission mode for Fe K edge with a Si(111) crystal monochromator. The Cu K edge XANES spectra were measured in the fluorescence mode with a Si(111) crystal monochromator. The spectra processing was also performed with Athena.

In Situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS).
The DRIFTS analysis was performed with a PerkinElmer Frontier FTIR spectrometer. For this analysis, 37−57 mg of sample was made into selfsupporting wafers. To remove surface contamination, the sample was heated up in 7% O 2 /He from 30 to 350°C with a ramp of 10°C/min. During this ramp, the spectra were recorded as background at different temperatures. After holding at 350°C for 30 min, the sample was cooled to 30°C and purged in He until complete O 2 removal. The sample was then exposed to 500 ppm of NO/He for 30 min during which the spectra were recorded. Then, the sample was purged with He for 10−15 min while recording the spectra. After purging, the sample was heated up in a He environment from 30 to 350°C with a ramp of 10°C/min.
The spectra were recorded in the range of 4400−500 cm −1 with a resolution of 2 cm −1 . The spectra at 30°C in He were used as background for NO adsorption and He purge data. The spectra recorded in O 2 /He were used as background for NO-TPD. All spectra were normalized by the sample weight.

Catalytic Performance Measurement.
The ammonia selective catalytic oxidation was evaluated in a fixed-bed flow reactor. The composition and flow rate of the inlet gas mixture were set by the mass flow controller. The typical reaction gas composition was 5000 ppm of NH 3 , 5 vol % O 2 , and balance He. The flow rate of the mixed gas was 100 mL/ min. Typically, 50 mg of catalyst was placed in the reaction tube, and the product was detected with the quadrupole mass spectrometer (MS) quantitative gas analyzer (Hiden Analytical, UK). The reaction was studied in the temperature range of 473 to 673 K. After reaching the steady state at each reaction temperature, the reaction was maintained for at least 30 min to measure the MS signals of the reactants (NH 3 and O 2 ) and products (N 2 , N 2 O, and NO).

DFT Calculations.
The spin-polarized DFT + U calculations were performed through the calculation software QuantumATK. 32 All the geometries were fully optimized by using the Perdew−Burke−Ernzerhof functional. All the electronic structures were calculated by using the Heyd− Scuseria−Ernzerhof hybrid functional. 33 The values of U were carefully chosen according to relevant paper, which are 7 and 4 eV for Cu atoms and Fe atoms, respectively. The Brillouin zone was sampled using a 9 × 9 × 9 Monkhorst−Pack 34 k-point mesh and 520 Hartrees cutoff energy for the primitive cell lattice optimization, while 1 × 1 × 1 k-points are employed for subsequent adsorption calculations. During the optimization, the convergence criteria were set to 0.03 eV/A and 10 −5 eV for force and energy, respectively. We chose the (0001) surfaces of Fe 2 O 3 and (111) surfaces of CuO owing to their lower surface energy and greater stability.
The adsorption energy per molecule was calculated as follows: where E surf + adsorbate is the total energy of the whole system which includes the gas molecule and the surface slab structure. E surf and E adsorbate are the energy of a sole surface slab and an isolated gas molecule, respectively. According to the equation, the negative adsorption energy indicates the existence of adsorption while the positive one means no evident adsorption interactions. Cu is uniformly dispersed on iron oxide support, as shown in the homogeneous Cu map alongside that of Fe (Figure 1b). The ksp of Fe(OH) 3 is 1.1 × 10 −36 , while the ksp of Cu(OH) 2 is 4.8 × 10 −20 . With 1 wt % CuO loading, the molar ratio of Fe and Cu is 98.6. Although Fe is much more than Cu, Cu starts to precipitate after most of Fe has precipitated. Therefore, most of the Cu sites in 1 wt % CuO−Fe 2 O 3 prepared by the coprecipitation method are on the surface of the catalyst. DFT calculations were performed to identify stable Cu structures on the (0001) surface of Fe 2 O 3 (Figure 1c). The Cu atom is coordinated with four O atoms, which is confirmed by the EXAFS results. The EXAFS analysis shows no Cu−Cu scattering for 1 wt % CuO−Fe 2 O 3 (Figure 1g,h and fitting results in Figure S5 and Table S1), suggesting the formation of atomic Cu−O−Fe; the detailed discussion is provided in Supplemental Note 1 and Figure S6.

Distribution and Structure of Copper over Metal
The Cu(II) oxidation state is confirmed with XANES, which shows the 1s → 3d transition for the Cu(II) d 9 configuration between 8976.2 and 8977.3 eV (Figure 1d,e). The absorption energy of 1s → 3d transitions decreases from 8977.2 eV for pure CuO to 8976.2 eV for 1 wt % CuO−Fe 2 O 3 , whereas that of the 1s → 4p z transition increases by 2.7 eV from CuO for 1 wt % CuO−Fe 2 O 3 (Figure 1f and Table S2). The calculated PDOS of Cu atoms further confirms that the unoccupied Cu 3d state of the Cu single site over Fe 2 O 3 is located slightly above the Fermi level, which is lower than that of CuO ( Figure  1i). A Bader charge analysis is performed to analyze the charge transfer between Cu(II) sites and Fe 3+ on the Fe 2 O 3 (0001) surface. Compared with pure copper oxide, atomic Cu sites on Fe 2 O 3 are more positively charged (Tables S3 and S4). According to the literature, such reduced 1s → 3d and increased 1s → 4p z transitions suggest the formation of atomic sites with strong interactions with the support that increase the energy of the lowest unoccupied molecular orbital (LUMO) and reduce the energy of the highest occupied molecular orbital (HOMO). 3 The Cu(II) site has the 3d 9 configuration, and the energy levels of the occupied 3d orbitals and unoccupied 4p orbitals are considered as HOMO and LUMO, respectively. The 1s → 3d transition toward the half-empty orbital of Cu(II) can reflect the HOMO of Cu(II). In addition, the 1s → 4p z transition of 1 wt % CuO−Fe 2 O 3 is very close to the 1s → 4p xy peaks (Figure 1e blue), indicating that the symmetry of Cu(II) has changed. 35 In comparison, CuO (Figure 1e black) and CuO clusters over Fe 2 O 3 ( Figure  S7) have been separated into 1s → 4p z and 1s → 4p xy peaks, which can be explained via the Jahn−Teller effect. 36 We hypothesize that in 1 wt % CuO−Fe 2 O 3 , Cu 2+ replaces the position of lattice Fe 3+ , forming Cu 2+ −O−Fe 3+ that changes the electronic structures of both Cu 2+ and O 2− .
To further prove this, 1 wt % CuO−Fe 2 O 3 prepared by impregnation has Cu species out of the lattice, showing separated 1s → 4p z and 1s → 4p xy peaks, which are different from that of the coprecipitated Cu. As shown in Figure S8, the 1s → 4p z peak of Cu in Cu−Fe 2 O 3 prepared by coprecipitation becomes a shoulder. This is due to the fact that some Cu 2+ are in the Fe 3+ location with a reduced Jahn−Teller effect. Although with the reduced Jahn−Teller effect, the Cu single sites are still in an elongated octahedral alignment. The two O in the z-axis are difficult to observe and fit in the R-space. The EXAFS spectrum of the impregnated sample is similar to that of the CuO standard, which is also different with coprecipitated samples ( Figure S8). This suggests the possibility of Cu agglomeration on the surface of the impregnated samples, which leads to a stronger Jahn−Teller effect.
Combining the XANES and EXAFS results, we confirm that atomic Cu sites with a lower 3d 9 energy level are formed on  Table S2) and are different from atomic Cu−O−Fe sites.

Catalytic Activity on NH 3 -SCO.
We compare the catalytic behavior of atomic Cu−O−Fe with that of the clusters. For pure oxides, a shift from high N 2 selectivity to high NO selectivity is observed along with an increase in NH 3 conversion ( Figure S11). This trend is consistent with the i-SCR mechanism. 37,38 Atomic Cu−O−Fe improves the NH 3 conversion on Fe 2 O 3 surfaces (Figures 2a and S12). With the lowest 1s → 3d transition energy, atomic Cu−O−Fe achieves 2 times higher NH 3 conversion compared with CuO clusters at 573 K and keeps 88% N 2 selectivity at 673 K with 5000 ppm of inlet NH 3 (Figure 2b,c). In comparison, the catalytic performance of physically mixed 1 wt % CuO + 99 wt % Fe 2 O 3 is not different from that of pure Fe 2 O 3 ( Figure S13). 1 wt % CuO−Fe 2 O 3 prepared by impregnation gave less NH 3 conversion and N 2 selectivity compared to the coprecipitated 1 wt % CuO−Fe 2 O 3 ( Figure S8c), suggesting that atomic Cu− O−Fe in the lattice promotes both oxidation ability and NO adsorption and thus improving the N 2 yield. Compared to other catalyst systems in the literature, 13,39−44 the atomic Cu− O−Fe catalyst achieved the highest N 2 productivity of Cubased catalysts, even higher than some noble metal catalysts between 523 and 623 K (Figure 2d). Under realistic NH 3 slip conditions (1000 ppm of NH 3 and a WHSV of 120 mL NH3 · h −1 ·g −1 ), 100% NH 3 conversion and 99% N 2 selectivity are

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Research Article achieved at 573 K (Figure 2e), suppressing most of the reported catalysts (Table S5). The atomic Cu−O−Fe catalyst also offers good stability for at least 100 h (Figure 2f) under a high WHSV, showing its potential toward replacing expensive Pt catalysts. As Fe 2 O 3 provides negligible NH 3 conversion at 473 K, the Cu-based turnover frequency (TOF) was then calculated. In general, the TOF reduces as the loading of Cu increases (Figure 2g). The HOMO level (1s → 3d), which reflects the interaction between Cu and supports, is negatively correlated with TOF in the NH 3 -SCO activity (Figure 2h). The atomic Cu−O−Fe with the lowest HOMO has the highest TOF of 5.9 h −1 , which is 56 and 16 times higher than that of pure CuO and CuO clusters (20 wt %) over Fe 2 O 3 . In addition to the different chemical environments of Cu, the agglomeration of CuO clusters at higher Cu loadings reduces the amount of accessible Cu, leading to the decrease of TOF. These results confirm our hypothesis that the catalytic performance can be modified significantly by forming atomic Cu−O−M sites.

Determination of Bridging O 2− in Cu−O−Fe and Its Impact on NH 3 Conversion.
Loading just 1 wt % of Cu(II) onto Fe 2 O 3 leads to 15 times higher conversion at 523 K (22.9 vs 1.5%), suggesting that atomic Cu−O−Fe is the major active species for NH 3 oxidation. The conversion of NH 3 to NO is the first step and the rate-determining step in NH 3 oxidation, which involves the Cu + /Cu 2+ redox and bridging O 2− removal/formation. The literature suggested that oxides with weak metal−oxygen bonds exhibit good redox properties and thus have higher rates of NO formation. 11 NAP-NEXAFS and in situ XAFS experiments are performed to identify bridging O 2− and its dynamics under NH 3 oxidation conditions.
The pre-edge of the O K-edge is separated into two peaks (1s to Fe 3d t 2g and e g and Cu 3d e g , ligand to metal charge transfer) due to the ligand-field splitting, which reflects the transitions to antibonding O 2p states hybridized with the 3d metal states (Figure 3g). 45 For pure α-Fe 2 O 3 , the ratio of 1s to t 2g and 1s to e g peak is about 1:1. 46 When atomic Cu is loaded over Fe 2 O 3 , the peak of 1s to t 2g (529.7 eV) becomes slightly higher than the peak of 1s to e g (531.0 eV), revealing the contribution from additional bridging O 2− (Cu−O−Fe). Such an influence is less when Cu 2+ is reduced due to the change of its 3d orbital geometry (Figure 3b,e) and less Cu−O bonds (the Cu−O coordination numbers reduce from 3.28 to 2.40 when switching from O 2 to NH 3 , Table S6). The decrease in the O K-edge spectra occurs along with the reduction in the Cu L 3 -edge spectra, confirming the removal of O in Cu−O−Fe upon Cu reduction. The O K-edge ΔNEXAFS between pure O 2 and pure NH 3 condition at 573 K and ΔNEXAFS between pure O 2 and 90% NH 3 condition at 673 K reveal the spectroscopy feature of the reactive bridging O 2− that can be taken away by NH 3 , with the major 1s to 2p−3d transition preedge at 529.4 eV (Figure 3h (Figure 3g). The lower 2p−3d energy is consistent with the decreased 3d 9 energy of the atomic Cu sites. Such weak Fe/Cu−O bonds explain the reactivity of bridging O 2− that can be easily taken away by NH 3 . Along with the removal of O 2− is the reduction of Cu 2+ to Cu + , as shown in the corresponding Cu L 3 -edge NAP-NEXAFS spectra (Figure 3c,f). The simultaneous O 2− removal and Cu 2+ reduction start from 10% O 2 + 90% NH 3 , 50% O 2 + 50% NH 3 , and 50% O 2 + 50% NH 3 at 473, 573, and 673 K, respectively ( Figures S14 and 3). At 573 K, only a trace amount of Cu 2+ exists under pure NH 3 (Figure 3c). At 673 K, all the Cu 2+/+ is reduced to Cu 0 with pure NH 3 and even surface Fe 3+ is reduced to Fe 2+ (Figure 3d,f; the bulk is still Fe 3+ as seen in Figure S15 We further compare the redox behaviors of atomic Cu sites and CuO clusters over Fe 2 O 3 by using the in situ XANES study at the Cu K-edge. The NH 3 -SCO reaction is carried out  Figure S16). We hypothesize that the lower 2p−4sp energy of Cu−O−Fe has a better energy match toward the nitrogen lone pair in NO (Figure 5d), contributing to the enhanced adsorption of NO, which leads to a better i-SCR rate (r2). The detailed discussion is provided in the Supplemental Note 2. This is investigated via a series of in situ characterization techniques and DFT calculations below.
The NAP-NEXAFS experiments confirm that the adsorption of NO is stronger than that of NH 3 and O 2 on 1 wt % CuO− Fe 2 O 3 . As shown in the N K-edge NAP-NEXAFS spectra, the surface-adsorbed NO (400.3 eV, Figure S17) cannot be removed by NH 3 and O 2 at room temperature for 1 wt % CuO−Fe 2 O 3 (Figure 5a). In comparison, oxygen can be replaced by ammonia. On changing from O 2 to NH 3 atmosphere, the surface-adsorbed O 2 over 1 wt % CuO− Fe 2 O 3 gradually weakens, and the surface-adsorbed NH 3 signal

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Research Article increases (402.1 eV, Figure S18) until the surface O 2 is completely replaced with NH 3 . The contribution of atomic Cu−O−Fe sites is further studied by in situ DRIFTS. At room temperature, NO is first adsorbed as bridged nitrate (1223 and 1604 cm −1 , Figure S19) on the surface of Fe 2 O 3 and 1 wt % CuO−Fe 2 O 3 , and then bridged nitrate is gradually transformed into monodentate nitrate at 1279 and 1518 cm −1 . 48,49 This phenomenon is more obvious on the surface of 1 wt % CuO−Fe 2 O 3 . An additional −OH group (3600−3800 cm −1 ) is formed over the 1 wt % CuO−Fe 2 O 3 surface. The possible reason is that Cu 2+ replaces Fe 3+ , so there will be more H + to neutralize the charge, leading to the high intensity of the −OH group. 50,51 The OH group over catalysts is removed under 300°C ( Figure S20). As the temperature rises, the monodentate nitrate on the Fe 2 O 3 surface is converted into bridged nitrate, bidentate nitrate (1551 and 1571 cm −1 ), and free NO 2 − ions 52 (1253 cm −1 ) ( Figure S21). Free NO 2 − will then lead to the formation of surface nitro species (1351 cm −1 ) ( Figure S21), 53 which are inactive in the SCR reaction due to their structure and thermal stability. 54 (Figure 5b,c). Therefore, the atomic Cu−O−Fe sites increase the surface NO density, prevent the formation of inactive nitro species, and improve the stability of the surface monodentate nitrate, which is the most active nitrate species in the SCR reaction. 58 The Fe 2 O 3 (0001) surface is reported to dominate under natural conditions, 59 and therefore, the structure of the α-  (Table S3), which is much lower than that of CuO (0.85 e, Table S4). After adsorption of NO, the electrons at O 2− (4) move toward NO (Table S8) removal performance, which increases the NH 3 conversion from 0 to 4% and 18 to 78% at 473 and 573 K, respectively, in comparison with lattice O 2− from pure Fe 2 O 3 . The in situ XAFS study indicates that the atomic Cu−O−Fe site shows better O 2− removal/ formation and Cu 2+ /Cu + redox performance than CuO clusters due to the highly active bridging O 2− in atomic Cu− O−Fe and thus has a better ability to oxidize NH 3 to NO in the first step (eq 1). Therefore, the weakened Cu−O−Fe bonding forms reactive O 2− that enables Cu redox, leading to a higher NH 3 oxidation activity compared with CuO clusters.
With a lower O antibonding orbital energy, we speculate that the O in Cu−O−Fe has better energy match with N in NO, leading to strong NO adsorption. In situ DRIFTS confirms the strong adsorption of nitrate species over 1 wt % CuO−Fe 2 O 3 , which suggests that the bridging O 2− in Cu−O− Fe offers unique NO adsorption via O−N. This is different from lattice O 2− in CuO (Cu−O−Cu) and Fe 2 O 3 (Fe−O− Fe). Therefore, the reactive bridging O 2− not only promotes the Cu 2+ /Cu + redox for NH 3 conversion but also enhances NO adsorption on the catalyst surface for subsequent N−N coupling, which explains the 24 and 28% of N 2 selectivity increase from pure Fe 2 O 3 at 573 and 673 K, respectively (Figure 2b,c).

CONCLUSIONS
Our study highlights the importance of bridging O 2− in catalysis at the atomic Cu−O−Fe site. Such reactive O 2− stems from the weak bonding with Cu and Fe, as confirmed in the O K-edge ΔNEXAFS and DFT calculations. As a result, the reactive O 2− can be easily taken away by NH 3 , promoting the rapid Cu+/Cu 2+ redox and NH 3 conversion. The reduced 2p− 3d antibonding orbital energy at the Cu−O−Fe site also leads to strong NO adsorption and acceleration of the N−N coupling to N 2 as the final product. Thus, the Cu−O−Fe sites show a 16 times higher activity than CuO clusters. The bridging O 2− in Cu−O−Fe directly adsorbs NO and prevents the formation of inactive nitro species, achieving 100% NH 3 conversion with 99% N 2 selectivity. Notably, tailoring the dband of atomic sites and its neighbor O 2− will be the key to selective oxidation reactions, which can be extended to other systems. Therefore, the ability to directly probe and design those bridging O 2− at the metal/support interface is crucial in catalyst design, mechanism study, and practical applications. ■ ASSOCIATED CONTENT * sı Supporting Information